Conventional fuel control systems utilizing fuel operated servo devices for actuating a positionable member such as a fuel metering valve or other engine control devices are well known in the art. It has been a common practice to extract from fuel pump output flow the required pressurized fuel flow for input to the servo and vent servo exhaust flow to a relatively low pressure fuel source such as the inlet of the fuel pump. Obviously such an arrangement wherein relatively high pressure fuel discharged by the fuel pump and used for servo control purposes undergoes a substantially large pressure drop in passage to the fuel pump inlet and resultant energy loss. Furthermore, the fuel pump capacity which generally dictates size and weight of the fuel pump must be selected to fulfill the fuel flow requirements of the engine. In addition, the increased fuel pump capacity necessitated by the servo means places a corresponding higher demand on power input to drive the fuel pump.
Reference is made to U.S. Pat. No. 3,521,447 to F. R. Rogers and D. L. Greenawalt for example of a known fuel control having a fuel operated servo network wherein the total output flow of a fuel pump is available for energizing fuel operated servo means as well as for fuel metering purposes to operate an engine. The subject matter of U.S. Pat. No. 3,521,447 represents a significant advance in the fuel control art in that it avoids the above-mentioned undesirable features normally found in fuel controls embodying fuel operated servo networks. The present invention represents an improvement over the apparatus shown and described in U.S. Pat. No. 3,521,447 in that the same desired results are obtained in a much simpler manner with less complicated structure.